This invention relates to devices designed to damp vibrations on large stiff structures.
One such structure is marine propulsion equipment which typically includes a steel subbase supporting turbines and a gearbox. In a modern subbase design, "cutouts" have been made to lower the weight. Unfortunately, the lighter design is conducive to increased vibrational levels and provides less surface area to apply damping treatments.
Prior art includes both tuned and untuned dampers. Tuned dampers have a natural frequency matched to the frequency of the vibrating structure. These devices are frequency specific, must be "retuned," and do not provide the needed vibration reduction over a broad frequency range.
Untuned dampers have a broad band or frequency independent vibrational response. A prior art untuned damper includes damping tiles used as broad band dampers comprising plates of a plastic damping material affixed to the surface of the structure. The dimension of the tile must be chosen in direct proportion to the thickness of the structure and therefore the tiles are applicable only to thin structures or structures which can accommodate a substantial addition of weight, which is a large fraction of the weight of the structure to be damped. Since the damping tile operates by absorbing a certain fraction of the structure's strain energy, the tiles must be located at places of high strain and can dissipate bending motions more effectively than other types of motions. Another embodiment of untuned damping mechanisms includes damping coatings comprising broad band damping treatments used on large surfaces of structures to reduce multi-frequency vibration. These coatings are not effective on the smaller, modern designs of the marine propulsion subbase.